This invention relates to pulse detection and, more particularly, to a pulse detector which accurately detects pulses read from a magnetic disk by a magnetic head which may exhibit residual magnetism. The invention also relates to a discriminator which uses the detected pulses to discriminate between offset errors and gain errors in the pulse detecting channel.
In conventional hard disk drives, servo information typically is recorded for the purpose of maintaining the head or heads of the drive in accurate registration with a track being scanned. In earlier hard disk drives comprised of multiple disks, it was conventional to dedicate the entire surface of one disk to servo information; and a separate servo head was used to read that information from which positional, or tracking errors were detected. A closed loop feedback arrangement adjusted the servo head relative to the servo tracks being scanned thereby so as to correct for tracking errors. Since the servo head was included in a stack of heads, tracking error correction of the servo head resulted in tracking error correction of all of the heads.
More recently, servo information has been disposed in limited portions of each track on each disk surface; and the same head normally used to read or write useful data is used to read this servo information. Typically, a servo pattern is recorded in a sector header, with headers being disposed uniformly in each track. Although various types of servo patterns have been proposed, their primary objective is to produce a signal which represents the magnitude and direction of a tracking error. One type of servo pattern produces a pair of positive pulses of intermediate amplitude followed by a single negative pulse of large magnitude when scanned. If the head drifts to one side of the track, the amplitude of one of the positive pulses exceeds the amplitude of the other, and the amplitude of the negative pulse is reduced. Hence, the direction of the tracking error is detected as a function of which positive pulse amplitude increased, and the amount of this error is detected as a function of the difference between the peaks of the positive pulses.
Another type of servo pattern is formed as two (or more) bursts of magnetic domains, both offset from the center of the track in opposite directions. Because of this equal offset, when the head is centered on the track, the pulses derived from one burst will be of equal amplitude to the pulses derived from the other burst. If the head drifts to one side of the track, the pulse amplitudes derived from one burst will be greater than the pulse amplitudes derived from the other. Thus, by determining which of the bursts results in greater pulse amplitudes, the direction of the tracking error is detected. Similarly, the magnitude of this tracking error is sensed as a function of the difference between those pulse amplitudes. Typically, amplitudes of the pulses derived from the bursts are sensed by peak detection. Since each magnetic domain results in a pulse pair, one being positive-going and the other being negative-going, the pulses first are full-wave rectified and then these full-wave rectified pulses are detected.
In the foregoing peak detection arrangement, it is expected that the positive excursions above a base line (or AC reference level) produced when a burst of magnetic domains is scanned is equal to the negative excursions below that base line. However, it has been found in practice that the head which reads the servo pattern may exhibit residual magnetism. As a result, the positive excursions above the base line differ from the negative excursions below. This residual magnetism may be thought of as a bias on one side of the head gap but not the other, and may be produced during a data write operation. Data normally is written on a track until a servo pattern is reached, whereafter the same head is changed over to its read mode to sense the servo pattern for the purpose of error correction. During this write-to-read transition, residual magnetization may remain as a function of the direction that flux last passed through the head, that is, the direction of the flux used to write the last piece of data.
As a result of this residual magnetization, the base line of the pulses derived from one burst is shifted, but a similar shift is not present in the pulses derived from the other burst. Because of the offset in the bursts, the residual magnetism at one end of the gap will have the effect of biasing the base line of the pulses derived from the burst that is offset in the direction of that end. A similar bias effect is not present in the pulses derived from the burst that is offset in the direction of the other end of the gap. Consequently, when the pulses are full-wave rectified, prior to detecting their peaks, as is conventional, the peaks detected from the pulses that have been biased above (or below) the base line will be greater than the peaks which are detected from the pulses that have not been so biased. Thus, even though the head may be centered accurately on a track, the peak voltage level detected from the pulses derived from one burst will differ from the peak voltage that is detected from the pulses derived from the other burst. This peak differential is interpreted erroneously as a tracking error. Hence, even though the head is in proper registration with a track, a "correction" will be made, with the result that the head now will be shifted into misregistration. This occurrence, which typically follows a write-to-read transition, is referred to as Write Induced Position Error, or WIPE.
Another difficulty encountered in typical disk drive operations is the general inability of a typical pulse detecting channel to discriminate between offset errors and gain errors. An offset error is produced when the preamplifier circuit normally included in the write channel, having been heated by a write current when writing data, generates an output offset different from that from the immediately preceding read operation. This new offset then is coupled into the read channel by a high pass filter. As a result, the pulses produced by the read channel exhibit a corresponding shift either upwardly or downwardly, depending upon the direction in which the offset is generated.
At a write-to-read transition, as when the servo pattern is sensed following a write operation, the gain of the playback amplifiers, and more particularly, the gain of the read channel, may need adjustment. For example, at the beginning of the servo pattern, the gain of the read channel may be too high and automatic gain control (AGC) operates to adjust this gain to its proper level. Conversely, if the gain of the read channel is too low at the beginning of the servo pattern, AGC operation increases the gain to its proper level.
The envelope of the pulses derived from, for example, the offset servo bursts, is used to detect both offset and gain errors. However, when the pulses are full-wave rectified, it often is difficult, if not impossible, to discriminate between gain and offset errors. Consequently, the proper corrective operation is not easily implemented.